Kotlin - Extension for final class

Is it possible to create extension of final classes like String? Like in swift it is possible to add additional methods inside a extension of final class.

For an example - I would like to create a method in String extension which will tell me String have valid length for password.

 val password : String = mEdtPassword!!.getText().toString()

 // how to define haveValidLength method in extension
 val isValid : Boolean = password.haveValidLength()

Note - That example is just for a sake to understand usability of extension, not a real scenario.

Solution

yes, you can. Kotin extension method provides the ability to extend a class with new functionality without having to inherit from the class or use any type of design pattern such as Decorator.

Below is an extension method for a String:

//  v--- the extension method receiver type
fun String.at(value: Int) = this[value]

And the extension method code generated as Java below:

public static char at(String receiver, int value){
    return receiver.charAt(value);
}

So an extension method in Kotlin is using delegation rather than inheritance.

Then you can calling an extension method like as its member function as below:

println("bar".at(1))//println 'a'

You also can write an extension method for the existing extension function, for example:

fun String.substring(value: Int): String = TODO()

//    v--- throws exception rather than return "ar"
"bar".substring(1)

But you can't write an extension method for the existing member function, for example:

operator fun String.get(value: Int): Char = TODO()

//   v--- return 'a' rather than throws an Exception
val second = "bar"[1]